[9] RFR(S): 8075136: Unnecessary sign extension for byte array access

Tobias Hartmann tobias.hartmann at oracle.com
Wed Mar 18 12:42:51 UTC 2015

On 18.03.2015 12:38, Andrew Haley wrote:
> On 03/18/2015 11:19 AM, Tobias Hartmann wrote:
>> On 18.03.2015 11:27, Andrew Haley wrote:
>>> On 03/18/2015 09:51 AM, Tobias Hartmann wrote:
>>>> Byte array access:
>>>>   0x0000007f7453d8b4: add	x10, x10, w1, sxtw
>>>>   0x0000007f7453d8b8: ldrsb	w0, [x10,#16]
>>>> Char array access:
>>>>   0x0000007f7cb6f8b4: add	x8, x10, #0x10
>>>>   0x0000007f7cb6f8b8: ldrh	w0, [x8,w1,sxtw #1]
>>> Do you know why these are different?  The second form is definitely
>>> preferable.
>> Yes, it's because we have no operand for the indirect-with-offset
>> (no scaling) case and therefore it is matched to 'addP_reg_reg_ext'.
>> For this reason I added the two new operands indIndexOffsetI2L and
>> indIndexOffsetI2LN. But even with those operands, it only works if
>> we reduce their cost below INSN_COST. Otherwise, 'AddP (AddP reg
>> (ConvI2L ireg)) off' is matched to 'addP_reg_reg_ext' (cost
> Aha!  That's the bug.  'addP_reg_reg_ext' should be 2*INSN_COST,
> along with all the ALU extend and/or shift instructions.  My
> bad, probably.

Okay, I assume you meant '1.9*INSN_COST' as it is used for 'addP_reg_reg_lsl'?

Here is the updated webrev:

It works fine:
  0x0000007f6d1c4a34: add	x8, x10, #0x10
  0x0000007f6d1c4a38: ldrsb	w0, [x8,w1,sxtw #0]


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